metric(consumer): Add a metric to track the size of individual spans (#6300)

We only have this metric for generic metrics but it would be useful to
have for spans to potentially help understand memory profiles of the
spans consumer.

Author: ayirr7

rust_snuba/src/processors/spans.rs

@@ -3,6 +3,7 @@ use std::str::FromStr;
 
 use anyhow::Context;
 use chrono::DateTime;
+use rust_arroyo::timer;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use uuid::Uuid;
@@ -22,6 +23,8 @@ pub fn process_message(
     let payload_bytes = payload.payload().context("Expected payload")?;
     let msg: FromSpanMessage = serde_json::from_slice(payload_bytes)?;
 
+    timer!("spans.messages.size", payload_bytes.len() as f64);
+
     let origin_timestamp = DateTime::from_timestamp(msg.received as i64, 0);
     let mut span: Span = msg.try_into()?;